This invention relates to the manufacture of ultrasound transducers and more particularly to the fabrication of piezoelectric transducer components of very fine pitch. Ultrasound devices, such as those employed in the medical ultrasound imaging market, typically employ piezoelectric ceramic materials such as lead-zirconate-titanate (PZT) to both emit and receive ultrasound waves. For reasons of formability and improved electroacoustic performance, it can be beneficial to employ a composite piezoelectric material rather than a monolithic slab of PZT. Composites typically consist of individual small pieces of PZT distributed within and isolated by a supporting epoxy or other polymeric plastic matrix material. The pieces of PZT usually consist of small strips or posts embedded in the passive pliable and acoustically lossy host matrix material.
In the case of embedded strips of piezoelectric material, the composite is referred to as "one-dimensional" and each embedded strip can be as much as a few acoustic wavelengths wide. However, the array transducers used in medical ultrasound applications require piezoelectric strips, posts or rods of aspect ratio (width to height) lower than 0.7. The piezoelements of such medical transducers oftentimes must be no wider than this to achieve acceptable sector, linear or vector type phased array transducer performance. Frequently one has to resort to the technique of subdicing the elements such that each element subpiece or subelement is no wider than the above requirement for the aspect ratio. One way to make such a device is to first "subdice" all the subelements on a fine pitch and then electrically gang two, three or even four or more of said subelements to form the macroscopic transducer elements which are on a coarser pitch. A two-dimensional piezoelectric device is formed by dicing or cutting a subelement in two orthogonal directions to form posts or rods rather than strips as formed by dicing in one direction only.
A common and convenient method for making a one-dimensional composite is to start with a monolithic slab of piezoelectric material and, using a dicing saw, cut slots, trenches or gaps therein. After cutting, the slots, trenches or gaps may be back-filled with polymeric matrix material such as an epoxy. A two-dimensional composite can be made by also cutting orthogonal slots. In this bidirectional material, one may choose to cut and fill each direction sequentially for ease of manufacture. After filling the slots, the exposed flat surfaces of the composite structure are ground and lapped, as necessary, and then metalized or electroded and repoled, if necessary. The resulting structure essentially comprises a semiflexible mat consisting of strips, posts or rods of piezoelectric material laterally encased by polymeric matrix material such as epoxy. The isolated strips, posts or rods (which are typically made of PZT) have their opposite exposed edges or ends in contact with the metalized or electroded surfaces.
Composite piezoelectric materials have been shaped and formed to achieve the mechanical focussing of ultrasound waves. Composites have also been made for use in special applications to provide improved electroacoustic characteristics compared to those obtainable with monolithic piezomaterial. Examples of such applications include annular arrays and mechanically scanned low-mass devices commonly employed in the medical ultrasound field.
To satisfy the need for higher and higher frequency transducers, it is necessary to find ways to make composites having a very fine pitch between adjacent PZT strips or posts. This is based upon the fact that the thickness must decrease as the operational ultrasonic wavelength decreases and the pitch should also decrease in order to satisfy the above-mentioned aspect ratio criterion.
At the present time, the dicing technology utilizes diamond-abrasive thin-foil blades which rotate at 30,000-60,000 RPM. However, the use of such blades has limitations with respect to cutting increasingly narrow trenches or kerfs at constant or even increasing depths which is necessary to produce finer pitch composites. Blades of 15 microns and less thickness are hard to work with and difficult to obtain. In addition such blades simply become mechanically unstable when used to cut very thin slots at great depths. In addition, as the ratio of cut depth/blade thickness (and kerf width) increases, the blade life is shortened, the kerf taper becomes unacceptable and the frequency of catastrophic blade failure increases. Obviously, if dicing is undertaken at a very fine pitch and the composite is of large macroscopic size, it is likely that a blade failure in the midst of dicing will ruin that part as a whole. A substantial value-added which has been invested to the point of failure is thereby lost to scrap. Thus, the difficulty of achieving narrower kerfs, or slots, limits or prohibits the manufacture of the finer pitch composites with high yield.